The Fletcher-Munson findings are well known in the prior art and generally teach that as the signal level of program material is lowered, the responsiveness of the human ear decreases. The result is that at low volume levels, the human ear is less able to hear bass sounds. Presently, many audio systems utilize a manual loudness control to boost low and high-end response at low volume levels in order to compensate for the decreased responsiveness of the human ear. Such a manual loudness control circuit is illustrated in U.S. Pat. No. 4,490,843.
Other circuits relate to improving the sound quality in automotive systems by implementing a controllable bass contour network coupled to the program source material. However, these circuits utilize notch filters for equalizing the frequency resonance within a vehicle's interior. These circuits also use a feedback circuit to detect and adjust bass levels. Such circuits are shown in U.S. Pat. Nos. 4,809,338 and 4,759,065.
Still other circuits provide automatic loudness compensation to bass boost at 60 Hz. One such circuit utilizes a 2:1 compressor so that the input signals can be compressed such a circuit is shown in U.S. Pat. No. 4,739,514.
It would be advantageous to have a circuit that automatically compensates for the decrease in program material at lower volumes by mimicking the Fletcher Munson curves using the r.m.s. (root mean square) value of an audio signal to boost bass within the 40–80 dB range. The circuit would be useful in restaurants, stores or high quality home stereo systems to boost the bass content of an input audio signal as the strength of the signal decreases.
In one embodiment, a circuit with characteristics of first order bass boost would automatically boost bass by altering the corner frequency of a circuit filter depending on the input signal. A method of providing an automatic loudness compensation circuit comprises receiving an input audio signal containing bass content, coupling the input audio signal to a voltage detector, coupling an output voltage of the voltage detector to a filter circuit, wherein the filter circuit adjusts a corner frequency associated with the filter such that the corner frequency is inversely related to the input audio signal to boost the bass content of the input audio signal, coupling an output of the filter circuit to a power amplifier for amplifying the filter circuit output, and driving an audio speaker with the amplified filter circuit output.
An apparatus according to the invention comprises a terminal for receiving an audio input signal, a power supply voltage having sufficient voltage to drive an audio output speaker, an R.M.S. detector for providing an R.M.S. voltage from the audio input signal, a variable low pass filter circuit for adjusting a corner frequency associated with the low pass filter such that the corner frequency is inversely related to the audio input signal and supplying an output signal which is increased as the audio input signal decreases, a power amplification stage for increasing the power of the output signal from the low pass filter circuit, and a terminal for providing an amplified output signal.
According to another aspect of the invention, there may be a digital signal processor (DSP) filter utilized such that in addition to altering the corner frequency, the DSP filter could also simulate varying the order of the circuit. By controlling the order of the filter circuit, the roll-off associated with the corner frequency could be adjusted to allow for a more accurate simulation of the Fletcher-Munson curves.